This invention relates to an "on-off" switchable permanent magnetic holding device for holding of or onto ferromagnetic objects.
A permanent magnetic holding device for holding ferromagnetic objects such as intended workpieces is already known, as disclosed in U.S. Pat. No. 4,055,824, which comprises at least two pairs of plate-shaped pole shoes of material with high magnetic permeability, each pole shoe pair being divided by a nonmagnetic medium, such as brass. Sandwiched between the two pairs of plate-shaped pole shoes are one or several plate-shaped rigid permanent magnets each having a corresponding centrally located circular recess in each of which a disc-shaped permanent magnet is turnably positioned. The rigid and the turnable magnets are magnetized to provide each magnet with two pole pairs of opposite magnetic polarity on each face thereof and with alternating polarity in axial direction, i.e., on the opposite flat sides of the magnets.
With the permanent magnets as thus arranged, in the switched on rotative position of the turnable magnet in the holding device, similar pole regions of the rigid and turnable permanent magnets, i.e. regions of the same magnetic polarity, lie adjacent to and magnetize the two magnetically divided pole shoes of each pole plate to provide a magnetic holding force thereat, while in the switched off rotative position of the turnable magnet dissimilar pole regions of the rigid and turnable magnets, having opposite magnetic polarity, lie adjacent to the pole shoes of each pole plate and thus leave them unmagnetized and devoid of any magnetic holding force.
With permanent magnetic holding devices as described above, at least two of the holding surfaces thereof, i.e., one pair of poles shoes, ordinarily remain unused as, for instance, because of the presence outwardly adjacent the faces of such pole shoe pair of an actuating or control knob for rotating the turnable permanent magnet. Furthermore, the holding surfaces of the rigid magnet are located diametrically outward relative to the turnable permanent magnets with this type of holding device, resulting in an unavoidably large structure height or dimension.